1. Field of the Invention
The present invention relates to an apparatus for driving an objective lens used in an optical head for optically recording information onto an optical disk and optically reading information therefrom, and particularly to an apparatus for driving an objective lens for inputting and outputting light with respect to the optical head.
2. Description of the Related Art
In an optical head for recording and reproducing information with respect to an optical disk such as a compact disk, an objective lens for inputting and outputting light is disposed so as to be opposed to an optical disk, and the objective lens is moved in a radius direction of the optical disk by an actuator, whereby a track on the optical disk is traced.
Furthermore, in order to regulate a focus position of the objective lens by moving the objective lens upward and downward in accordance with the up and down movement of the optical disk surface caused by warpage thereof, to correct a tracking shift of the objective lens caused by eccentricity of the optical disk, and to regulate a relative tilt angle between the optical disk and the objective lens, the objective lens is moved by an apparatus for driving an objective lens.
In the apparatus for driving an objective lens, the objective lens is moved in three directions: a vertical direction (focusing direction) of the optical disk, a radius direction (tracking direction) of the optical disk, and a tangent direction of the optical disk, thereby enabling the optical head to exactly trace a track on the optical disk.
A conventional apparatus for driving an objective lens is described in, for example, Japanese Laid-Open Publication No. 4-319537. FIG. 12 is a schematic view thereof. In this figure, an objective lens 101 is held by a lens holder 102, and the lens holder 102 is connected to an elastic supporting plate 103 via four wires 104. The lens holder 102 is elastically supported by the elastic supporting plate 103 by using each wire 104, so that the lens holder 102 can be moved in any of a focusing direction F, a tracking direction T, and a tangent direction K of an optical disk with respect to the elastic supporting plate 103.
A pair of electromagnetic coils 106a and 106b are provided on a base 105. The elastic supporting plate 103 is fixed to the base 105, and the lens holder 102 is disposed between the electromagnetic coils 106a and 106b. By appropriately regulating an electromagnetic force of the electromagnetic coils 106a and 106b, the lens holder 102 is moved in the focusing direction F, the tracking direction T, and the tangent direction K of an optical disk.
A pair of slits 103a are formed in the elastic supporting plate 103, thereby providing a pair of supporting portions 103b. Resonance of each wire 104 is suppressed by an elastic force of the supporting portions 103b. 
In recent years, there has been a demand for a thinner apparatus for driving an objective lens of the above-mentioned type, capable of stably positioning an objective lens with high precision. In order to satisfy such a demand, an apparatus for driving an objective lens has been variously modified and developed. The process thereof will be described below.
As shown in FIG. 13, a basic structure of an apparatus 200 for driving an objective lens of the above-mentioned type includes a lens holder 202 for holding an objective lens 201, a base 203, and a plurality of wires 204 connecting the lens holder 202 to the base 203. A light beam 205 passes through the objective lens 201 and is reflected by a prism 206 to be input and output between an optical head (not shown) and an optical disk (not shown). In this structure, the height of the apparatus 200 for driving an objective lens is added to the height of the prism 206, so that the optical head becomes bulky, which is not preferable.
Therefore, in an apparatus 300 for driving an objective lens shown in FIG. 14, considering that the height of a lens holder 302 is larger than a distance between the respective wires 304 at a connected portion between the lens holder 302 and each wire 304, an objective lens 301 is disposed away from the connected portion so as to make the lens holder 302 thinner at a portion of the objective lens 301, and a prism 306 is disposed at a portion where the lens holder 302 is thinner. Thus, the optical head is made thinner.
In the structure shown in FIG. 14, the objective lens 301 is disposed away from the connected portion between the lens holder 302 and each wire 304. Therefore, when resonance occurs in each wire 304, a vibration amplitude of the objective lens 301 which is disposed away from the connected portion becomes large, degrading displacement frequency characteristics of the apparatus for driving an objective lens. Furthermore, resonance around an axis in the tracking direction T (i.e., resonance in a direction represented by an arrow A) occurs in the vicinity of a cutoff frequency of a gain of a servo driving system which moves the objective lens 301 in the focusing direction F. This makes focus control by the servo driving system unstable.
Furthermore, in the case where the objective lens 301 is moved in a radial tilt direction which is a direction around an axis in the tangent direction K of an optical disk, as well as the focusing direction F and the tracking direction T, each wire 304 is greatly twisted. Therefore, unnecessary resonance is likely to occur, further degrading the displacement frequency characteristics of the objective lens 301. Furthermore, a diameter, a length, and the like of each wire 304 are not determined so as to suppress unnecessary resonance of each wire 304, but determined by a lower order resonance frequency of an apparatus for driving an objective lens. Therefore, an unnecessary resonance frequency of each wire 304 cannot be shifted by changing the diameter, length, and the like of each wire 304.
Thus, in an apparatus 400 for driving an objective lens shown in FIG. 15, an elastic supporting portion 406 is provided on a base 403, and one end of each wire 404 is fixed to the elastic supporting portion 406, whereby vibration of the elastic supporting portion 406 is further suppressed by a damping member (not shown). In this structure, a resonance frequency of each wire 404 is shifted by the elastic supporting portion 406, whereby a resonance amplitude of each wire 404 is decreased. Because of this, the displacement frequency characteristics of an objective lens 401 are enhanced. Furthermore, a resonance frequency of each wire 404 can be removed from the vicinity of a cutoff frequency of a gain of a servo driving system which moves the objective lens 401 in the focusing direction F, without affecting a lower order resonance frequency of the apparatus 400 for driving an objective lens, whereby focus control by the servo driving system can be made stable.
However, in the apparatus 400 for driving an objective lens shown in FIG. 15, a printed substrate made of synthetic resin is used for the elastic supporting portion 406 in order to improve the ease of assembly. The printed substrate""s characteristics (e.g., coefficient of elasticity) change remarkably depending upon temperature and time. Therefore, a resonance frequency, a resonance amplitude, and the like of each wire 404 cannot be made stable.
Furthermore, in the case where a distance between the respective wires 404 is decreased in the focusing direction F so as to make the apparatus 400 for driving an objective lens thinner, a tilt of an optical axis of the objective lens 401 when the objective lens 401 is moved is increased for the following reason. When a distance between the respective wires 404 is decreased in the focusing direction F, a radius of a path of the objective lens 401 when the objective lens 401 is moved in the focusing direction F becomes smaller, as a result of which a tilt becomes too large to be negligible. Furthermore, the elastic supporting portion 406 may move in the tangent direction K along with deformation of each wire 404. In this case, a tilt is further increased.
The occurrence of such a tilt will be described with reference to FIGS. 16 to 18. Under the situation shown in FIG. 16, the objective lens 401 stays at an initial position, and each wire 404 and the elastic supporting portion 406 are not deformed.
In the case where the objective lens 401 is moved upward in the focusing direction F, as shown in FIG. 17, a movement path of a lens holder 402 has a generally elliptical shape, and a tilt occurs in the objective lens 401. As described above, as a distance between the respective wires 404 becomes smaller, a radius of a movement path of the lens holder 402 becomes smaller and a tilt becomes larger. The lens holder 402 tilts in the direction represented by an arrow N5 with the movement of the lens holder 402. At this time, forces in directions represented by arrows P5 and P6 occur in each wire 404, and the elastic supporting portion 406 pivots in the direction represented by an arrow M5 with respect to a pivot axis S by the forces in the directions represented by the arrows P5 and P6. As a result, the lens holder 402 further tilts in the direction represented by an arrow N6, so that a tilt is further increased.
In the case where the objective lens 401 is moved downward in the focusing direction F as shown in FIG. 18, the lens holder 402 also tilts in the direction represented by an arrow N7 with the movement of the lens holder 402, in the same way as shown in FIG. 17. At this time, forces in directions represented by arrows P7 and P8 occur in each wire 404, and the elastic supporting portion 406 pivots in the direction represented by an arrow M6 with respect to the pivot axis S by the forces in the directions represented by the arrows P7 and P8. As a result, the lens holder 402 further tilts in the direction represented by an arrow N8, so that a tilt is further increased.
When a tilt is increased as described above, optical aberration on a recording surface of an optical disk is increased; as a result, a signal to be recorded onto the optical disk and a signal to be reproduced therefrom are remarkably degraded.
An apparatus for driving an objective lens of the present invention, includes: a base which is moved along an optical disk: an objective lens disposed so as to be opposed to the optical disk and having an optical axis in a vertical direction with respect to the optical disk, for optically recording information onto the optical disk or optically reproducing information from the optical disk; a holder portion for holding the objective lens; a plurality of elastic arms extending between the base and the holder portion in a tangent direction of the optical disk, for supporting the holder portion movably with respect to the base; and a driving portion for generating an electromagnetic force for moving the holder portion with respect to the base, wherein the elastic arms are disposed away from each other in a vertical direction, the base includes each movable portion for supporting one end of each of the elastic arms, and each of the movable portions is supported at each position in a vertical direction outside of one end of each of the elastic arms, so as to be elastically pivotable around each axis in a radius direction of the optical disk.
In one embodiment of the present invention, each of the movable portions of the base includes a damping member for suppressing vibration of each of the movable portions.
In another embodiment of the present invention, the base includes an elastic plate, the elastic plate is bent at each position in a vertical direction outside of one end of each of the elastic arms, and each of the movable portions of the base is supported so as to be elastically pivotable around each corner of the bent elastic plate.
In another embodiment of the present invention, each axis of the movable portions in a radius direction of the optical disk is elastically movable in a tangent direction of the optical disk.
In another embodiment of the present invention, each of the elastic arms is in the shape of a bar.
In another embodiment of the present invention, the base includes an elastic plate made of metal having each of the movable portions of the base, a stationary section made of metal, and a resinous holding portion for holding the elastic plate, and the holding portion is integrated with the elastic plate and the stationary section by molding.
In another embodiment of the present invention, the base includes an elastic plate made of metal having each of the movable portions of the base, each of the elastic arms is made of metal and soldered to each of the movable portions of the elastic plate, and a reverse surface of each of the movable portions of the elastic plate to which each of the elastic arms is connected is surface-treated for the purpose of decreasing solder wettability.
In another embodiment of the present invention, the surface treatment for the purpose of decreasing solder wettability is Ni-plating.
In another embodiment of the present invention, the base includes an elastic plate made of metal having each of the movable portions of the base, each of the elastic arms is made of metal and soldered to each of the movable portions of the elastic plate, and each of the movable portions of the elastic plate to which each of the elastic arms is connected is surface-treated for the purpose of increasing solder wettability.
In another embodiment of the present invention, the surface treatment for the purpose of increasing solder wettability is Au-plating.
In another embodiment of the present invention, the base includes an elastic plate made of metal having each of the movable portions of the base, a stationary section made of metal, and a resinous holding portion for holding the elastic plate, the holding portion is integrated with the elastic plate by molding, and the elastic plate is soldered to the stationary section.
In another embodiment of the present invention, each of the movable portions of the base is made of synthetic resin integrally molded with at least part of the base.
Thus, the invention described herein makes possible the advantage of providing an apparatus for driving an objective lens which is adapted for a thin type apparatus, and is capable of realizing stable displacement frequency characteristics by suppressing resonance, preventing a tilt of an objective lens from increasing with the movement in a focusing direction and even canceling a tilt of the objective lens at all times.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.